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Food allergies or food intolerances affect nearly everyone at some point.
People often have an unpleasant reaction to something they ate and wonder
if they have a food allergy. One out of three people either say that they
have a food allergy or that they modify the family diet because a family member
is suspected of having a food allergy. But only about three percent of children
have clinically proven allergic reactions to foods. In adults, the prevalence
of food allergy drops to about one percent of the total population.
This difference between the clinically proven prevalence of food
allergy and the public perception of the problem is in part due to
reactions called "food intolerances" rather than food allergies. A food
allergy, or hypersensitivity, is an abnormal response to a food that is
triggered by the immune system. The immune system is not responsible for
the symptoms of a food intolerance, even though these symptoms can
resemble those of a food allergy.
It is extremely important for people who have true food allergies to
identify them and prevent allergic reactions to food because these
reactions can cause devastating illness and, in some cases, be fatal.
An allergic reaction involves two features of the human immune
response. One is the production of immunoglobulin E (IgE), a type of
protein called an antibody that circulates through the blood. The other is
the mast cell, a specific cell that occurs in all body tissues but is
especially common in areas of the body that are typical sites of allergic
reactions, including the nose and throat, lungs, skin, and
The ability of a given individual to form IgE against something as
benign as food is an inherited predisposition. Generally, such people come
from families in which allergies are commonónot necessarily food allergies
but perhaps hay fever, asthma, or hives. Someone with two allergic parents
is more likely to develop food allergies than someone with one allergic
Before an allergic reaction can occur, a person who is predisposed to
form IgE to foods first has to be exposed to the food. As this food is
digested, it triggers certain cells to produce specific IgE in large
amounts. The IgE is then released and attaches to the surface of mast
cells. The next time the person eats that food, it interacts with specific
IgE on the surface of the mast cells and triggers the cells to release
chemicals such as histamine. Depending upon the tissue in which they are
released, these chemicals will cause a person to have various symptoms of
food allergy. If the mast cells release chemicals in the ears, nose, and
throat, a person may feel an itching in the mouth and may have trouble
breathing or swallowing. If the affected mast cells are in the
gastrointestinal tract, the person may have abdominal pain or diarrhea.
The chemicals released by skin mast cells, in contrast, can prompt hives.
Food allergens (the food fragments responsible for an allergic
reaction) are proteins within the food that usually are not broken down by
the heat of cooking or by stomach acids or enzymes that digest food. As a
result, they survive to cross the gastrointestinal lining, enter the
bloodstream, and go to target organs, causing allergic reactions
throughout the body.
The complex process of digestion affects the timing and the location of
a reaction. If people are allergic to a particular food, for example, they
may first experience itching in the mouth as they start to eat the food.
After the food is digested in the stomach, abdominal symptoms such as
vomiting, diarrhea, or pain may start. When the food allergens enter and
travel through the bloodstream, they can cause a drop in blood pressure.
As the allergens reach the skin, they can induce hives or eczema, or when
they reach the lungs, they may cause asthma. All of this takes place
within a few minutes to an hour.
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In adults, the most common foods to cause allergic reactions include:
shellfish such as shrimp, crayfish, lobster, and crab; peanuts, a legume
that is one of the chief foods to cause severe anaphylaxis, a sudden drop
in blood pressure that can be fatal if not treated quickly; tree nuts such
as walnuts; fish; and eggs.
In children, the pattern is somewhat different. The most common food
allergens that cause problems in children are eggs, milk, and peanuts.
Adults usually do not lose their allergies, but children can sometimes
outgrow them. Children are more likely to outgrow allergies to milk or soy
than allergies to peanuts, fish, or shrimp.
The foods that adults or children react to are those foods they eat
often. In Japan, for example, rice allergy is more frequent. In
Scandinavia, codfish allergy is more common.
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If someone has a life-threatening reaction to a certain food, the
doctor will counsel the patient to avoid similar foods that might trigger
this reaction. For example, if someone has a history of allergy to shrimp,
testing will usually show that the person is not only allergic to shrimp
but also to crab, lobster, and crayfish as well. This is called
Another interesting example of cross-reactivity occurs in people who
are highly sensitive to ragweed. During ragweed pollination season, these
people sometimes find that when they try to eat melons, particularly
cantaloupe, they have itching in their mouth and they simply cannot eat
the melon. Similarly, people who have severe birch pollen allergy also may
react to the peel of apples. This is called the "oral allergy syndrome."
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A differential diagnosis means distinguishing food allergy from food
intolerance or other illnesses. If a patient goes to the doctor's office
and says, "I think I have a food allergy," the doctor has to consider the
list of other possibilities that may lead to symptoms that could be
confused with food allergy.
One possibility is the contamination of foods with microorganisms, such
as bacteria, and their products, such as toxins. Contaminated meat
sometimes mimics a food reaction when it is really a type of food
There are also natural substances, such as histamine, that can occur in
foods and stimulate a reaction similar to an allergic reaction. For
example, histamine can reach high levels in cheese, some wines, and in
certain kinds of fish, particularly tuna and mackerel. In fish, histamine
is believed to stem from bacterial contamination, particularly in fish
that hasn't been refrigerated properly. If someone eats one of these foods
with a high level of histamine, that person may have a reaction that
strongly resembles an allergic reaction to food. This reaction is called
Another cause of food intolerance that is often confused with a food
allergy is lactase deficiency. This most common food intolerance affects
at least one out of ten people. Lactase is an enzyme that is in the lining
of the gut. This enzyme degrades lactose, which is in milk. If a person
does not have enough lactase, the body cannot digest the lactose in most
milk products. Instead, the lactose is used by bacteria, gas is formed,
and the person experiences bloating, abdominal pain, and sometimes
diarrhea. There are a couple of diagnostic tests in which the patient
ingests a specific amount of lactose and then the doctor measures the
body's response by analyzing a blood sample.
Another type of food intolerance is an adverse reaction to certain
products that are added to food to enhance taste, provide color, or
protect against the growth of microorganisms. Compounds that are most
frequently tied to adverse reactions that can be confused with food
allergy are yellow dye number 5, monosodium glutamate, and sulfites.
Yellow dye number 5 can cause hives, although rarely. Monosodium glutamate
(MSG) is a flavor enhancer, and, when consumed in large amounts, can cause
flushing, sensations of warmth, headache, facial pressure, chest pain, or
feelings of detachment in some people. These transient reactions occur
rapidly after eating large amounts of food to which MSG has been added.
Sulfites can occur naturally in foods or are added to enhance crispness
or prevent mold growth. Sulfites in high concentrations sometimes pose
problems for people with severe asthma. Sulfites can give off a gas called
sulfur dioxide, which the asthmatic inhales while eating the sulfited
food. This irritates the lungs and can send an asthmatic into severe
bronchospasm, a constriction of the lungs. Such reactions led the U.S.
Food and Drug Administration (FDA) to ban sulfites as spray-on
preservatives in fresh fruits and vegetables. But they are still used in
some foods and are made naturally during the fermentation of wine, for
There are several other diseases that share symptoms with food
allergies including ulcers and cancers of the gastrointestinal tract.
These disorders can be associated with vomiting, diarrhea, or cramping
abdominal pain exacerbated by eating.
Gluten intolerance is associated with the disease called
gluten-sensitive enteropathy or celiac disease. It is caused by an
abnormal immune response to gluten, which is a component of wheat and some
Some people may have a food intolerance that has a psychological
trigger. In selected cases, a careful psychiatric evaluation may identify
an unpleasant event in that person's life, often during childhood, tied to
eating a particular food. The eating of that food years later, even as an
adult, is associated with a rush of unpleasant sensations that can
resemble an allergic reaction to food.
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To diagnose food allergy a doctor must first determine if the patient
is having an adverse reaction to specific foods. This assessment is made
with the help of a detailed patient history, the patient's diet diary, or
an elimination diet.
The first of these techniques is the most valuable. The physician sits
down with the person suspected of having a food allergy and takes a
history to determine if the facts are consistent with a food allergy. The
doctor asks such questions as:
- What was the timing of the reaction? Did the reaction come on quickly,
usually within an hour after eating the food?
- Was allergy treatment successful? (Antihistamines should relieve hives,
for example, if they stem from a food allergy.)
- Is the reaction always associated with a certain food?
- Did anyone else get sick? For example, if the person has eaten fish contaminated
with histamine, everyone who ate the fish should be sick. In an allergic
reaction, however, only the person allergic to the fish becomes ill.
- How much did the patient eat before experiencing a reaction? The severity
of the patientís reaction is sometimes related to the amount of food the
- How was the food prepared? Some people will have a violent allergic reaction
only to raw or undercooked fish. Complete cooking of the fish destroys those
allergens in the fish to which they react. If the fish is cooked thoroughly,
they can eat it with no allergic reaction.
- Were other foods ingested at the same time of the allergic reaction? Some
foods may delay digestion and thus delay the onset of the allergic reaction.
Sometimes a diagnosis cannot be made solely on the basis of history. In
that case, the doctor may ask the patient to go back and keep a record of
the contents of each meal and whether he or she had a reaction. This gives
more detail from which the doctor and the patient can determine if there
is consistency in the reactions.
The next step some doctors use is an elimination diet. Under the
doctor's direction, the patient does not eat a food suspected of causing
the allergy, like eggs, and substitutes another food, in this case,
another source of protein. If the patient removes the food and the
symptoms go away, the doctor can almost always make a diagnosis. If the
patient then eats the food (under the doctor's direction) and the symptoms
come back, then the diagnosis is confirmed. This technique cannot be used,
however, if the reactions are severe (in which case the patient should not
resume eating the food) or infrequent.
If the patient's history, diet diary, or elimination diet suggests a
specific food allergy is likely, the doctor will then use tests that can
more objectively measure an allergic response to food. One of these is a
scratch skin test, during which a dilute extract of the food is placed on
the skin of the forearm or back. This portion of the skin is then
scratched with a needle and observed for swelling or redness that would
indicate a local allergic reaction. If the scratch test is positive, the
patient has IgE on the skin's mast cells that is specific to the food
Skin tests are rapid, simple, and relatively safe. But a patient can
have a positive skin test to a food allergen without experiencing allergic
reactions to that food. A doctor diagnoses a food allergy only when a
patient has a positive skin test to a specific allergen and the history of
these reactions suggests an allergy to the same food.
In some extremely allergic patients who have severe anaphylactic
reactions, skin testing cannot be used because it could evoke a dangerous
reaction. Skin testing also cannot be done on patients with extensive
For these patients a doctor may use blood tests such as the RAST and
the ELISA. These tests measure the presence of food-specific IgE in the
blood of patients. These tests may cost more than skin tests, and results
are not available immediately. As with skin testing, positive tests do not
necessarily make the diagnosis.
The final method used to objectively diagnose food allergy is
double-blind food challenge. This testing has come to be the "gold
standard" of allergy testing. Various foods, some of which are suspected
of inducing an allergic reaction, are each placed in individual opaque
capsules. The patient is asked to swallow a capsule and is then watched to
see if a reaction occurs. This process is repeated until all the capsules
have been swallowed. In a true double-blind test, the doctor is also
"blinded" (the capsules having been made up by some other medical person)
so that neither the patient nor the doctor knows which capsule contains
The advantage of such a challenge is that if the patient has a reaction
only to suspected foods and not to other foods tested, it confirms the
diagnosis. Someone with a history of severe reactions, however, cannot be
tested this way. In addition, this testing is expensive because it takes a
lot of time to perform and multiple food allergies are difficult to
evaluate with this procedure.
Consequently, double-blind food challenges are done infrequently. This
type of testing is most commonly used when the doctor believes that the
reaction a person is describing is not due to a specific food and the
doctor wishes to obtain evidence to support this judgment so that
additional efforts may be directed at finding the real cause of the
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At least one situation may require more than the simple ingestion of a
food allergen to provoke a reaction: exercise-induced food allergy. People
who experience this reaction eat a specific food before exercising. As
they exercise and their body temperature goes up, they begin to itch, get
light-headed, and soon have allergic reactions such as hives or even
anaphylaxis. The cure for exercised-induced food allergy is simpleónot
eating for a couple of hours before exercising.
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Food allergy is treated by dietary avoidance. Once a patient and the
patient's doctor have identified the food to which the patient is
sensitive, the food must be removed from the patient's diet. To do this,
patients must read lengthy, detailed ingredient lists on each food they
are considering eating. Many allergy-producing foods such as peanuts,
eggs, and milk, appear in foods one normally would not associate them
with. Peanuts, for example, are often used as a protein source and eggs
are used in some salad dressings. The FDA requires ingredients in a food
to appear on its label. People can avoid most of the things to which they
are sensitive if they read food labels carefully and avoid
restaurant-prepared foods that might have ingredients to which they are
In highly allergic people even minuscule amounts of a food allergen
(for example, 1/44,000 of a peanut kernel) can prompt an allergic
reaction. Other less sensitive people may be able to tolerate small
amounts of a food to which they are allergic.
Patients with severe food allergies must be prepared to treat an
inadvertent exposure. Even people who know a lot about what they are
sensitive to occasionally make a mistake. To protect themselves, people
who have had anaphylactic reactions to a food should wear medical alert
bracelets or necklaces stating that they have a food allergy and that they
are subject to severe reactions. Such people should always carry a syringe
of adrenaline (epinephrine), obtained by prescription from their doctors,
and be prepared to self-administer it if they think they are getting a
food allergic reaction. They should then immediately seek medical help by
either calling the rescue squad or by having themselves transported to an
emergency room. Anaphylactic allergic reactions can be fatal even when
they start off with mild symptoms such as a tingling in the mouth and
throat or gastrointestinal discomfort.
Special precautions are warranted with children. Parents and caregivers
must know how to protect children from foods to which the children are
allergic and how to manage the children if they consume a food to which
they are allergic, including the administration of epinephrine. Schools
must have plans in place to address any emergency.
There are several medications that a patient can take to relieve food
allergy symptoms that are not part of an anaphylactic reaction. These
include antihistamines to relieve gastrointestinal symptoms, hives, or
sneezing and a runny nose. Bronchodilators can relieve asthma symptoms.
These medications are taken after people have inadvertently ingested a
food to which they are allergic but are not effective in preventing an
allergic reaction when taken prior to eating the food. No medication in
any form can be taken before eating a certain food that will reliably
prevent an allergic reaction to that food.
There are a few non-approved treatments for food allergies. One
involves injections containing small quantities of the food extracts to
which the patient is allergic. These shots are given on a regular basis
for a long period of time with the aim of "desensitizing" the patient to
the food allergen. Researchers have not yet proven that allergy shots
relieve food allergies.
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Milk and soy allergies are particularly common in infants and young
children. These allergies sometimes do not involve hives and asthma, but
rather lead to colic, and perhaps blood in the stool or poor growth.
Infants and children are thought to be particularly susceptible to this
allergic syndrome because of the immaturity of their immune and digestive
systems. Milk or soy allergies in infants can develop within days to
months of birth. Sometimes there is a family history of allergies or
feeding problems. The clinical picture is one of a very unhappy colicky
child who may not sleep well at night. The doctor diagnoses food allergy
partly by changing the child's diet. Rarely, food challenge is used.
If the baby is on cow's milk, the doctor may suggest a change to soy
formula or exclusive breast milk, if possible. If soy formula causes an
allergic reaction, the baby may be placed on an elemental formula. These
formulas are processed proteins (basically sugars and amino acids). There
are few if any allergens within these materials. The doctor will sometimes
prescribe corticosteroids to treat infants with severe food allergies.
Fortunately, time usually heals this particular gastrointestinal disease.
It tends to resolve within the first few years of life.
Exclusive breast feeding (excluding all other foods) of infants for the
first 6 to 12 months of life is often suggested to avoid milk or soy
allergies from developing within that time frame. Such breast feeding
often allows parents to avoid infant-feeding problems, especially if the
parents are allergic (and the infant therefore is likely to be allergic).
There are some children who are so sensitive to a certain food, however,
that if the food is eaten by the mother, sufficient quantities enter the
breast milk to cause a food reaction in the child. Mothers sometimes must
themselves avoid eating those foods to which the baby is allergic.
There is no conclusive evidence that breast feeding prevents the
development of allergies later in life. It does, however, delay the onset
of food allergies by delaying the infant's exposure to those foods that
can prompt allergies, and it may avoid altogether those feeding problems
seen in infants. By delaying the introduction of solid foods until the
infant is 6 months old or older, parents can also prolong the child's
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There are several disorders thought by some to be caused by food
allergies, but the evidence is currently insufficient or contrary to such
claims. It is controversial, for example, whether migraine headaches can
be caused by food allergies. There are studies showing that people who are
prone to migraines can have their headaches brought on by histamines and
other substances in foods. The more difficult issue is whether food
allergies actually cause migraines in such people. There is virtually no
evidence that most rheumatoid arthritis or osteoarthritis can be made
worse by foods, despite claims to the contrary. There is also no evidence
that food allergies can cause a disorder called the allergic tension
fatigue syndrome, in which people are tired, nervous, and may have
problems concentrating, or have headaches.
Cerebral allergy is a term that has been applied to people who have
trouble concentrating and have headaches as well as other complaints. This
is sometimes attributed to mast cells degranulating in the brain but no
other place in the body. There is no evidence that such a scenario can
happen, and most doctors do not currently recognize cerebral allergy as a
Another controversial topic is environmental illness. In a seemingly
pristine environment, some people have many non-specific complaints such
as problems concentrating or depression. Sometimes this is attributed to
small amounts of allergens or toxins in the environment. There is no
evidence that such problems are due to food allergies.
Some people believe hyperactivity in children is caused by food
allergies. But researchers have found that this behavioral disorder in
children is only occasionally associated with food additives, and then
only when such additives are consumed in large amounts. There is no
evidence that a true food allergy can affect a child's activity except for
the proviso that if a child itches and sneezes and wheezes a lot, the
child may be miserable and therefore more difficult to guide. Also,
children who are on anti-allergy medicines that can cause drowsiness may
get sleepy in school or at home.
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One controversial diagnostic technique is cytotoxicity testing, in
which a food allergen is added to a patient's blood sample. A technician
then examines the sample under the microscope to see if white cells in the
blood "die." Scientists have evaluated this technique in several studies
and have not been found it to effectively diagnose food allergy.
Another controversial approach is called sublingual or, if it is
injected under the skin, subcutaneous provocative challenge. In this
procedure, dilute food allergen is administered under the tongue of the
person who may feel that his or her arthritis, for instance, is due to
foods. The technician then asks the patient if the food allergen has
aggravated the arthritis symptoms. In clinical studies, researchers have
not shown that this procedure can effectively diagnose food allergies.
An immune complex assay is sometimes done on patients suspected of
having food allergies to see if there are complexes of certain antibodies
bound to the food allergen in the bloodstream. It is said that these
immune complexes correlate with food allergies. But the formation of such
immune complexes is a normal offshoot of food digestion, and everyone, if
tested with a sensitive enough measurement, has them. To date, no one has
conclusively shown that this test correlates with allergies to foods.
Another test is the IgG subclass assay, which looks specifically for
certain kinds of IgG antibody. Again, there is no evidence that this
diagnoses food allergy.
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Controversial treatments include putting a dilute solution of a
particular food under the tongue about a half hour before the patient eats
that food. This is an attempt to "neutralize" the subsequent exposure to
the food that the patient believes is harmful. As the results of a
carefully conducted clinical study show, this procedure is not effective
in preventing an allergic reaction.
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Food allergies are caused by immunologic reactions to foods. There
actually are several discrete diseases under this category, and a number
of foods that can cause these problems.
After one suspects a food allergy, a medical evaluation is the key to
proper management. Treatment is basically avoiding the food(s) after it is
identified. People with food allergies should become knowledgeable about
allergies and how they are treated, and should work with their physicians.
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Food Allergy and Anaphylaxis
American College of Allergy, Asthma and Immunology
Asthma and Allergy Foundation of America
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